Many implantable medical electronic devices utilize an internal source of electrical energy to power the device electronics for the purposes of, for example, diagnostics and/or therapy. Additionally, many implantable devices require such a significant amount of power that it is necessary to utilize transcutaneous energy transmission (TET) from an extracorporeal source to an implanted receiver which is connected to a rechargeable battery. To date, one of the more efficient recharging means employs an external transmission coil and an internal receiver coil which are inductively coupled. In this TET approach, the external primary transmission coil is energized with alternating current (AC), producing a time varying magnetic field that passes through the patient's skin and induces a corresponding electromotive force in the internal secondary receiving coil. The voltage induced across the receiving coil may then be rectified and used to power the implanted device and/or charge a battery or other charge storage device. Additionally, many medical electronic devices rely on noninvasive telemetry in order to allow data and control signals to be bi-directionally communicated between the implanted medical device and an external device or system. Such telemetry can be accomplished via a radio frequency (RF) coupled system using a transmitting antenna to a receiving antenna by way of a radiated carrier signal, or by using the power transfer coils for data transmission.
Electronic circuits and systems that are to be implanted in living organisms are hermetically packaged in a biocompatible material for the purposes of protecting the electronic circuitry from body fluids and protecting the organism from infection or other injury caused by the implanted materials. The most commonly used materials for implantable electronic devices are biocompatible metals, glass, and ceramics. Biocompatible metals include, for example without limitation, titanium, a titanium alloy, stainless steel, cobalt-chromium, platinum, niobium, tantalum, and various other possible alloys. Normally, metal enclosures consist of separate metal parts welded together to insure hermeticity. However, implant enclosures made of conductive metal present difficulties with respect to both transcutaneous energy transmission and telemetry. Specifically, the time varying magnetic charging field induces eddy currents within the metal housing and inhibits the magnetic flux as it passes through the case. With respect to RF telemetry from the implanted device to a receiver external to the patient, the metal case acts as a Faraday cage and tends to limit the rate of information transfer between the implanted device and the external system due to circulating eddy currents that absorb energy from the magnetic field and produce a magnetic field that opposes the incident magnetic field. The magnitude of the eddy currents is approximately proportional to the frequency of the AC magnetic field because the magnitude of the voltage induced within the conductive material is proportional to the time rate of change of magnetic flux as described in Faraday's Law, E=−dΦ/dt, where E is the induced voltage and Φ is the magnetic flux impinging on the material. The carrier frequency for telemetry is limited by the amount of eddy current attenuation that the system can tolerate.
It is necessary to transmit significant amounts of power through the device case in order to recharge the device battery in a reasonable period of time. The implanted induction charging system typically uses a two-winding transformer with a non-ferrous (air) core. The energy transfer efficiency is approximately proportional to the number of turns in the transformer windings and the rate of change (frequency) of the alternating current, as follows:e2=M di1/dt+L2 di2/dt 
Where e2 is the voltage induced across the secondary winding, M is the mutual inductance of the primary and secondary windings, L2 is the inductance of the secondary winding and di1/dt and di2/dt are the time rate of change (frequency) of the primary and secondary currents.
Because the physical size of the implanted device limits the size and, hence, the inductance (L2) of the receiving coil within the device, it is desirable to operate the inductive coupling system at the highest possible frequency in order to obtain the maximum coupling efficiency and energy transfer. Raising the operating frequency, however, increases the eddy current losses, so that the overall induction system efficiency is severely reduced. Additionally, such induced eddy currents create unwanted heat within the implantable enclosure.
A number of approaches have been proposed to address the limitations of induced eddy currents upon a metallic medical device enclosure with respect to TET and telemetry systems:
Ceramic Sleeve with a Metal Header. One approach is to utilize a deep drawn ceramic sleeve forming the majority of the enclosure body. The sleeve has a closed end, an open end for receiving electronic components and a metallic header for closing the open end (see U.S. Pat. No. 4,991,582.) Such a device, when implanted, has ceramic distal, proximal and side walls (relative to the skin) and an extracorporeal charging and/or telemetry device. This approach has, however, primarily been limited to small medical device enclosures (e.g., cochlear implants) due to the weight of the ceramic material. For larger devices such as an implantable pulse generator, the weight of the ceramic sleeve becomes a significant limitation due to the overall weight of the enclosure given the amount of ceramic used, the relatively large density of the ceramic, and the required large wall thickness (see also U.S. Pat. No. 6,411,854).
Polymer Casing. Another approach is to avoid using both metal (problematic due to eddy currents) and ceramic (problematic due to weight) in favor of a biocompatible polymer material for the outer enclosure. This approach attempts to use epoxy to encapsulate the receiving coil, antenna, and a secondary enclosure and provide a hermetically sealed sub-housing for the system electronics. The polymer and/or epoxy material does not, however, provide for a true hermetic seal, as eventually body fluids migrate through the material and degrade the receiving coil and antenna.
External Coil. In order to circumvent the problem of the metal housing material reducing the efficiency of the TET induction system efficiency, some devices have opted to place the receiving induction coil on the outside of the metal housing. This approach, however, increases both the size of the implant, the complexity of the surgical implant process, and the complexity of the device given the necessity for additional hermetic electrical feed-through connections between the secondary coil and the internal electronic circuitry. Additionally, the external coil would still have to be a biocompatible material as with the polymer casing approach above.
Thin Metal Window. U.S. Pat. No. 7,174,212 presents an approach for increasing the efficiency of high speed/high carrier frequency telemetry via the use of (1) a metallic housing having a thin metal telemetry window having a thickness on the order of 0.005 inches and/or (2) a metal alloy (e.g., titanium alloy) window having reduced electrical conductivity parameters. However, as the window material still is made of an electrically conductive material (although reduced in thickness), this solution is non-ideal as an RF telemetry signal and/or a magnetic field will still induce eddy currents thereby reducing the efficiency of the telemetry link.
Machined Grooves in Metal Casing. U.S. Pat. No. 5,913,881 presents an approach for increasing the efficiency of high speed/high carrier frequency telemetry by creating grooved recesses arranged on either or both sides of the implanted housing wall to reduce the overall thickness of the wall and to create discontinuities along the wall surface in order to reduce the conductivity of the metal housing wall, thereby decreasing the induced eddy currents and providing increased telemetry efficiency.
Other hermetic housings for implantable medical devices are described in U.S. Pat. No. 4,785,827 and U.S. Pat. No. 5,876,424.
Improved medical device structures and methods of manufacture are needed to overcome at least the shortcomings stated above.